Transplantation of insulin-producing tissue is known to restore carbohydrate tolerance to normal in insulin-requiring diabetics. Two potential drawbacks of this procedure include requirements for immunosuppressive therapy and difficulties involved in procurement of viable human donor pancreatic tissue. Protection of transplanted beta cells by artificial membranes permeable to glucose and insulin but impermeable to larger agents such as antibodies and lymphocytes is one possible approach to overcoming these two difficulties. Such semipermeable membranes can be fabricated in the form of capillaries and tubes of various diameters. An artificial endocrine pancreas consisting of beta cells cultured on bundles of these fibers has been successfully maintained in vitro in our laboratory for several weeks by perfusion with tissue culture medium. These devices restore blood glucose homeostasis to normal when implanted ex vivo as arteriovenous shunts in the vascular system of alloxan diabetic rats. We intend to examine some of the problems involved in developing a device such as this into an implantable organ for long-term treatment of human diabetics.